Prior art battery packs such as disclosed in Patent Document #1 are per se known for use as secondary cell systems for which high capacity is required, such as systems for electric automobiles, railroad engines, power storage systems for electrical power systems, and so on. These battery packs include series cell groups in which a large number of secondary cells are connected in series, and are made by connecting a plurality of these series cell groups in parallel.
On the other hand, in Patent Document #2, for a battery pack in which a plurality of blocks are connected in series, with a plurality of secondary cells being connected in parallel in each parallel block, it is proposed to detect the amount of change of the voltage of each parallel block before and after charging and discharging, and to determine the presence or absence of faulty cells on the basis of the results of detection.